Transmembrane Domain Dominance Drives Emergent Signaling and Allosteric Inversion in mGlu_1/5 Heterodimers

Class C GPCRs function as obligate dimers in which only one G protein can engage the complex at a time, but how each protomer contributes to heterodimer coupling has remained unresolved. Using CODA-RET, a BRET-based assay reporting direct Gα_q recruitment to defined, full-length receptor pairs, we show that signaling at the mGlu₁/₅ heterodimer flows predominantly through the mGlu₁ protomer; domain-swapped chimeras localize this dominance to the transmembrane domain. The dominance generates emergent signaling: cis-acting mGlu₁ PAMs and NAMs undergo allosteric inversion when coupling is restricted to mGlu₅. By contrast, the mGlu₅-selective NAM MTEP is silent at the heterodimer, mirroring mGlu₅'s minimal role in driving Gα_q. Because the mGlu₁ PAM tested acts only in cis, a trans-acting mGlu₁ PAM would theoretically be selective for mGlu₁/₁ homomers. These findings open a pharmacological design space in which protomer target and cis-versus-trans mode of action tune selectivity across mGlu₁/₁, mGlu₅/₅, and mGlu₁/₅ dimers.